I can just explain why this happened, up to you how to understand it and i hope it can help.Heat is useless energy, however air contain a heat that cant make any small differences of pressure because your casing is open system . First  actually its about heat transfer law and integration of material vibration .  Air intake from outside either  inside the casing,where act u want sink the hot air? or you want use the fan for cooling process.Act any casing fan behave as air circulation for most electronic part except the blower one have potency to cool your electronic part from temp 0 to ~~ be constant. The noise is coming from normal forced air conduction,convection why forced? because you use a fan, when use a fan it is not natural.How it make the casing side panel absorb vibration, you think what the material is tempered for the casing side panel? .You can touch your side panel,the material is not strong enough to store your fan vibration energy, You can try it ,if u stop the air intake using your hand the noise level will drop dramatically.you must know every mechanical unit like casing fan spinning  horizontally or vertically axis have a different level of noise. this is not fan problem or casing.Act you can still using same side panel and fan with silent condition. To counter the physical behavior or your material because or air flow through the fan blade going inside or outside is not important.See the fan frame is fixed with a screw or nut.However the fan is bearing mechanical in operated in 1000rpm - 3000rpm is not operates at good condition, the bearing hold the blades and spinning to make small scale. when its turning on more less 0.09 sec small noise until the inconstant fan operation operated at maximum point the vibration start making distribute to other material attached to it + small  potential energy .i hope you will find the answer on your own, that make you ill be better.

If put outside for sure not more noise but how about the wayering ? lol